The present invention relates to a measuring apparatus for detecting the distance "h" to an object, the apparatus comprising a transmission circuit for transmitting in the direction of the object a wave exhibiting frequency-modulated periods, a receive circuit for receiving the wave reflected by the object, a mixer circuit for forming a beat frequency wave between the transmitted wave and the received wave and a processor element processing beat frequency wave samples for supplying the information of the distance "h" to the indicator.
Such devices are widely used in the field of radio navigation for measuring the altitude, the object then being the ground. Other applications can, for example, be the measuring of the height of the metal level in a blast-furnace.
An apparatus of this type is described in the U.S. Pat. No. 4,568,938.
This apparatus which directly performs a Fourier transform of the beat frequency wave samples is not very suitable for measuring a small distance. Actually, to avoid that in each period of the frequency modulation, the phase jumps of the beat signal distort the spectrum evaluated by the Fourier transform, only the samples of the beat signal obtained within a single modulation period are to be considered. To determine the small distances one is forced to take a high slope of the frequency modulation. A high slope variation cannot last very long and results in the processing of only a small number of samples. In this case, the Fourier transform proves to be inefficient.